The present invention relates to gas sensors, and more particularly, but not exclusively to gas sensors incorporating a self-test capability. These types of gas sensors are hereinafter referred to as self-test gas sensors. The invention is suitable for use in electrochemical gas sensors, such as, for example, carbon monoxide (CO) sensors.
Conventional electrochemical gas sensors for use in CO sensors, operate by oxidizing a gas to be detected at a sensing electrode, thereby generating an electric current. The rate of access to the sensing electrode may be determined by a diffusion barrier, and the rate at which the electrode is capable of oxidizing the gas is arranged to be very much greater than the rate at which the gas can diffuse through the barrier. Generally the rate of oxidation, and hence the electric current generated, is controlled mainly by diffusion. This diffusion rate has a value (for a given gas concentration) and the sensor can be calibrated when it is manufactured. If the actividty of the sensing electrode falls with time, e.g., through poisoning, then the level of current generated eventually becomes limited by a lower oxidation rate. This results in a decrease in sensitivity of the sensor. There is then no way of determining, from the sensor output alone, whether the gas concentration is low, or whether the gas concentration is high and the electrode has lost activity become less sensitive.
Previously, so as to overcome this problem, the sensitivity of sensors was ascertained by regular tests involving exposure to a calibration gas of known concentration. In many situations, for example in a domestic carbon monoxide safety monitor, this was undesirable.
In the Applicant""s published International Patent Application WO-A-9703372, the contents of which are incorporated herein by way of reference, there is described a self-test gas sensor having electrolytic gas generation electrodes and sensor electrodes being located in close proximity one to the other within the same housing. The aforementioned gas sensor has several advantages over previous gas sensors, including lower operating power, as the amount of gas needed to be generated for the self test procedure is reduced; and lower assembly costs, as the test and sensing cells are effectively assembled in the same operation. The self-test gas sensor operates by generating locally a small volume of gas (hereinafter referred to as a xe2x80x9ctest gasxe2x80x9d) and detecting the test gas in the sensor, thereby confirming the status of the sensor, i.e. functioning or faulty.
It is desirable that the response of the gas sensor, during the self test procedure, should result only from the response of the sensor to the test gas generated during that procedure, and not arise from electrical interference which may occur between the sensor and gas generator circuits as a result of electrochemical reactions.
According to the present invention there is provided a self-test gas sensor comprising: a sensor for sensing a gas and a test cell, the test cell being arranged to generate a test gas on demand, and a test gas pathway for directing the test gas to the gas sensor so that generated test gas is detected by the gas sensor, thereby verifying that the gas sensor is functioning, characterised in that a baffle is disposed between the gas sensor and the test cell so as to prevent electrical interference therebetween.
Sensing electrodes are electrically isolated from test cell electrodes by way of the baffle. Preferably the baffle is situated in a fluid pathway between the sensing circuit and the test circuit. The baffle prevents flow of ions (and therefore electric current) between what is effectively a test cell and a gas sensor cell when the test cell is operating. The baffle may comprise two or more portions. The first portion preferably comprises a gas impermeable substrate, and the second portion comprises a gas permeable substrate. The gas permeable substrate of the baffle permits the passage of water vapour between the test cell and the gas sensor. The gas permeable substrate helps to maintain electrolyte, present in the gas sensor, at a substantially constant pH and/or concentration.
A further requirement is that the complexity of the sensor and gas generator cells, and their respective operating circuits, should be minimised in order to reduce unit costs.
According to another aspect of the invention a self test gas sensor includes first, second and third electrodes, the first electrode being a common electrode, the second electrode in operation with the first electrode, acting as a pair to generate a test gas; the third electrode in operation with the first electrode acting as gas sensor, characterized in that means is provided to isolate test and sensing electrodes one from another, so that only one pair of electrodes is operational at any instant.
It is also envisaged that an embodiment of the sensor having three electrodes is within the scope of the present invention, a counter electrode being common to a test and a sensing electrode. In this embodiment electrical interference or cross-talk between components is avoided by ensuring the sensing and test components operate at different instants. Cross talk between the two sets of electrodes is avoided by a switch which ensures that the test electrode and reference electrode are only energized when the sensing electrode is switched out of circuit. Clearly in an embodiment where there is a common electrolyte there is a risk of cross talk between two (or more) pairs of electrodes and switching one pair out of circuit whilst a separate pair are switched in circuit is one way of avoiding this.
Preferably a capacitive element is provided for storing energy from an energy source which supplies electric current to a pair of sensing electrodes during operation of the sensor, there being a switch arranged to disconnect the supply of electric current to the sensing electrodes and connect the capacitive element to the test electrodes thereby providing an independent source of current thereto.
In normal sensing use, the capacitive element is changed from the sensing circuit power supply. In test mode, the capacitor is isolated from the power supply by a switch, and connected to the test electrode circuit, so as to discharge through the test circuit and generate gas at the test electrode. The capacitor provides a supply isolated from the sensor circuit supply, and so the generation current will not flow through the sensor electrodes, provided also that a baffle as aforementioned is in place between the test electrode and sensing electrode.
Preferably the sensor and the test cell are disposed within a housing, which permits gas from the environment to pass to the gas sensor via a gas pathway. The gas pathway is preferably separated from a test gas pathway.
Preferably the electrically conductive pathways by-pass the or each baffle thereby further reducing the risk of interference between a test circuit and a sensing circuit.
There may be a counter electrode common to the sensor and the test cell. The sensor electrodes and the test cell electrodes may be formed on a gas permeable membrane, such as PTFE.
Strengthening ribs may be incorporated into the body of the housing. A diffusion barrier may be provided to limit the rate of arrival of a gas at the gas sensor.
UK Patent Application GB-A-2323171 (City Technology Limited) discloses a sensor in which electrical contact to an internal electrode is made, via an electrical connector, from an external terminal. The electrical connection is made between the electrical connector and the external terminal by forcing electrically conductive components together under pressure and maintaining the pressure throughout the working life of the sensor. It is believed that such connections eventually fail, not as a result of relaxation of the compression of components, but rather as a result of relaxation or perishing of either intervening gaskets or O-ring seals;
A further advantage of the present invention, over the arrangement described in GB-A2323171, is the fact that relatively few components are present compared to the arrangement shown in FIG. 1 of GB-A-2323171. This feature makes the present invention cheaper and easier to manufacture. Also, because less components are present there is less risk that the sensor will fail. This is particularly important when considering the working life of the sensor may be several years.
According to a different aspect of the present invention there is provided a self test gas sensor comprising a housing, which is hermetically sealed from an external atmosphere apart from at least one entrance, through which a gas to be sensed may pass, which sensor. in use contains an electrolyte in contact with a sensing electrode, a counter electrode and a test electrode, there being a wick interposed between the sensing and counter electrodes, the wick being dimensioned and arranged so as to supply electrolyte to both electrodes irrespective of the orientation of the sensor, and electrically conductive pathways provided which contact the counter and sensing electrodes and pass to electrical terminals supported on an external surface of the housing.
A filter means may also be provided, the filter means prevents unwanted substances, such as ethanol (C2H5OH), from contacting the sensor. The filter means may include a charcoal filter.
When a test gas is generated by the test cell, it passes to a recessed volume, from where it passes to the gas sensor. The recessed volume, may be situated in the housing. Most preferably, gas generated by the test cell passes to the gas sensor via a gas permeable membrane. The gas permeable membrane may be made of PTFE. The sensor electrodes and the test cell electrodes may be formed on a gas permeable membrane, such as PTFE.
The sensor and the test cell have an electrolyte so that, in use, an electrochemical reaction is supported between two test electrodes, and between the sensor electrodes. Means may be provided for maintaining the electrolyte in contact with the electrodes. The sensor and the test cell are preferably in fluid communication one with another, so that when the self-test gas sensor is in use, electrolyte may flow freely from the sensor to the test cell.
Electrodes are preferably operated by a potentiostat circuit. Separate voltage sources may be used to operate the test cell electrodes and the sensor cell electrodes. At least one reference electrode may be provided in contact with the electrolyte for use in the sensing circuit.
A barrier may be provided to prevent gas from the atmosphere from contacting the reference electrode via the recessed volume. The barrier may be formed integrally with the housing.
In many electrochemical processes it is advantageous to design an electrochemical cell so that electrolyte remains in intimate contact with electrodes, in varying conditions of orientation and movement of the cell and differing concentrations of electrolyte. This is especially so when the electrolyte volume varies with time and so occupies a varying proportion of a space between two or more electrodes, or between an electrode and an ambient atmosphere. This change in volume of electrolyte can give rise to variation in the effective area of electrodes which are in contact with the electrolyte. This area of contact should be maximised, or at least vary to the least degree possible.
A particular example of an electrochemical cell where the aforementioned problem has been experienced is in gas sensors. Gas sensors need to operate in any orientation so that the electrolyte concentration remains in equilibrium with the humidity of the atmosphere. Variation in volume and in effective area of contact in the manner previously described is therefore undesirable.
Various methods of overcoming the aforementioned problem exist for gas sensors. Examples include gelled electrolytes, which adhere to the surface of electrodes; solid polymer electrolytes, such as NAFION (Trade Mark); which can be cast onto or impregnated into the electrode surface; and wicks which are held against the surface of an electrode using physical pressure. All these however, suffer from disadvantages. Gelled electrolytes have a volume and consistency which often varies considerably with concentration, and so can flow from the electrode surface in high humidity atmospheres, solidifying elsewhere when the humidity falls again Solid polymer electrolytes remain in place, but are expensive, have a conductivity that varies strongly with humidity and so need liquid acid in contact with them. Also they can be difficult to apply sparingly to the electrode surface in high volume production. Wicks are cheap and effective, but careful mechanical design is needed to ensure that they remain in good physical contact with the electrode in any orientation and under conditions where the cell might suffer shock or impact. Also, in certain types of electrochemical cell, it is necessary to ensure that the electrode surface is entirely covered with electrolyte in order to prevent gas access to that surface. To ensure this using only physical pressure on a wick is difficult.
According to another aspect of the present invention there is provided an electrode assembly comprising a porous structure in contact with an electrode, the porous structure being arranged to adhere to the electrode so that in use the structure is in contact with an electrolyte, thereby continuously wetting substantially all the surface of the electrode with electrolyte.
Preferably the porous structure includes a wick material. The porous structure is preferably deposited onto a support substrate, which may be the electrode, in a liquid or paste-like form, for example by screen printing, thereby fabricating the electrode assembly. Fabrication is then completed by one or more of: drying, setting, sintering or pressing the porous structure adjacent the electrode so as to define the electrode assembly.
In a preferred embodiment a thin layer of wick material of fibrous or bound porous particular material is placed upon onto a wet deposited electrode surface and urged into a top layer of the electrode. Then using post-processing (for example by one or more of heat, pressure and drying), the wick material becomes firmly and uniformly attached to the electrode surface in such a way as to ensure uniform distribution of electrolyte over the surface without disrupting the bulk of the electrode structure. Thus electrical conductivity is ensured, the electrochemical efficiency of the electrode is unaltered and the porous structure is capable of supporting efficient transfer of liquid by capillary action.
The porous structure is advantageously dimensioned and arranged so that it comprises different layers or regions of differing porosity, so that layers close to the surface of the electrode have a greater affinity to the electrolyte than those layers further from the electrode surface. This enhances capillary action and improves the wicking effect, thus ensuring the electrode surface is always wetted by electrolyte.
An additional advantage, which may be of importance in certain sensors, is that gas diffusion electrodes need to maintain hydrophobicity through their bulk to maximize their reaction efficiency. Portions of the surface of the electrode assembly may be substantially hydrophobic, with some small hydrophilic areas. Generally electrodes have an hydrophilic surface in order to provide high surface area for reactions to proceed. The introduction of an hydrophilic wick material into the electrode assembly assists this process.
Preferably the porous wick material is sinteied onto the electrode at a temperature between 300xc2x0 C. to 370xc2x0 C. and most preferably within a temperature range between 320xc2x0 C. to 370xc2x0 C. The exact temperature depends upon the nature of ink printed onto the electrode and the substrate.
The electrode assembly may be incorporated into an electrochemical cell or a gas sensor, which may or may not be a self test gas sensor as herein before described.
Conventional electrochemical gas sensors comprise at least three electrodes, namely a sensing electrode, a reference electrode and a counter electrode, located within a housing containing electrolyte. The housing usually has a diffusion barrier in the form of a small aperture through which ambient gases can diffuse to contact the sensing electrode. The ambient gas are oxidized by the sensing electrode thereby generating an electrical current indicative of the concentration of oxidized gases. The rate of access of the ambient gas to the sensing electrode is determined by the design of the diffusion barrier and the rate at which the electrode can oxidize the ambient gases is arranged to be very much greater than the rate at which the gas diffuses through the barrier. Therefore the rate of oxidation of the gases, and hence the current generated, is controlled solely by the rate of diffusion, (and this is a known value for each sensor for a given gas concentration), when the sensor is manufactured. If the activity of the sensing electrode falls with time, for example, due to poisoning, then the current generated becomes limited by the lower oxidation rate at the sensing electrode and the sensitivity of the sensor falls. The sensor is not fail safe. There is no way of telling from the cell output whether the gas concentration is low, or that the concentration is higher and the sensing electrode has lost actvity.
Reliability of such sensors can be ascertained by regular tests involving exposure of the sensor to an external calibration gas. In many situations, for example, in a domestic CO safety monitor, this is not practical and is undesirable. To overcome this shortfall, it is known to construct sensors with a self-test ability which may be triggered remotely or locally.
GB-A-1 552 538 (Bayer) describes a self-test sensor assembly consisting of two parts, a sensor and a gas generation means, for example an electrolysis cell, joined by a delivery channel. Test gas is delivered directly to the sensing electrode of the sensor, with a membrane between the point of gas delivery and the outside world. Delivery is by a piston, a pressure difference resulting from the generation of gas itself, or other means. Signal gas enters the sensor from the atmosphere via the membrane. In this arrangement the concentration of test gas seen by the sensing electrode depends on the balance of the rate of generation of the gas and the rate of loss through the membranexe2x80x94the latter depends on the conditions (air flow) outside the membrane. As the generator is remote from the sensing electrode, there is a large volume to be filled with gas in order to ensure that a consistent known concentration is reached. This means the design is likely to require significant power, which is a limitation of the use of such a principle in a low power domestic monitor circuit.
GB-A-2 245 711 describes a gas sensor with solid electrolyte layers disposed on two sets of electrodes, one designed for a gas sensing function, and the other set provided for a test function. The test function electrodes are arranged to sense a gas normally present in the atmosphere, e.g., oxygen. A decrease in the signal from the test electrodes is taken to indicate a either a decrease in activity of the test electrodes, or a decrease in the permeability of the solid electrolyte, through which test and signal gas must pass before they reach the electrodes. Such change in permeability is a major factor in the performance of the sensor type disclosed in GB-A-2 245 711. The test of electrode decay rests on the assumption that the test electrodes will decay in the same way as the sensing electrodes. The test reaction uses oxygen (O2) and is fundamentally different from the sensing reaction for oxidizing gases, being a reduction rather than an oxidation reaction, and so this form of test is likely to prove unreliable. A test where the sensing electrodes oxidizes test gas generated in known quantity, as in GB-A-1 552 538 would be advantageous.
The Applicants Co-pending UK Patent Application No 9625463 discloses a self-test gas sensor including a housing containing at least a sensing electrode, a counter electrode and a test electrode. The sensor has the sensing and counter electrodes in a first electrolyte and the test electrode in a second electrolyte. Gas from the environment flows to the sensing electrode through a diffuser passage. In operation in a normal mode of operation, electrical potentials are applied to the electrodes for detecting when a gas to be sensed is present at the sensing electrode. In a test mode of operation, electrical potentials are applied to the electrodes so that the test electrode generates a gas which flows to the sensing electrode to enable an indication whether the sensor is operating correctly.
According to a yet further aspect of the present invention there is provided a sensor comprising a housing in which there is located a sensing electrode, a counter electrode, a reference electrode and electrolyte in contact with the electrodes, said housing having a diffusion barrier through which ambient gas to be detected may pass, the cell being operable in a sensing mode, where electrical potentials are applied to the counter electrode and the sensing electrode, to effect reaction of ambient gases that reach the sensing electrode and thereby produce an electrical current indicative of the concentration of the gas to be detected, characterised in that the sensing electrode comprises an electrically conductive layer deposited on a first gas permeable substrate, said counter electrode and the reference electrode each comprise an electrically conductive layer deposited on a surface of the second gas permeable substrate which faces towards the sensing electrode, and a gas permeable wick which in use conveys electrolyte to the said electrodes is positioned between the substrates in contact with the electrodes.
A cheap and accurate means is provided of self-testing, wherein the test gas is generated within of the sensor and in a controlled amount by application of a suitable voltage potential.
The proposed self-test electrochemical cell described in the Applicant""s UK Patent Application No 9625461 includes a planar arrangement of one or more sensing electrodes and one or more electrolytic gas generator electrodes in the same housing in contact with common or separate electrolytes, with associated counter and reference electrodes. The gas generating electrodes are located close to the sensing electrode or electrodes, so as to minimise the amount of gas that is needed to be generated to effect a test of the operation of the sensor. In one embodiment described in our aforementioned UK Patent Application the test gas Hydrogen H2 is delivered to the sensing electrode in the gas phase, by evolution into a communicating space above the electrodes. The test gas generated by the generator electrode is H2 which is generated by the reaction:
2H++2exe2x86x92H2(gas)xe2x80x83xe2x80x83(Eqn 1)
It is a feature of this reaction that, if the generation electrode has a source of oxygen available, then oxygen will be reduced also:
xc2xdO2+2H+++2exe2x86x92H2Oxe2x80x83xe2x80x83(Eqn 2)
which passes a current in parallel with that passed in the H2 generation reaction, and so reduces the operating efficiency of the generator. Oxygen reduction reaction occurs particularly advantageously if the generator electrode is permeable and in contact with the atmosphere. Conventionally, the gas generator electrode is made of a material which is an active catalyst for the generation of H2 such as for example platinum, and the electrical current generated at the sensing electrode as a result of the reduction of oxygen will be large compared with that generated as a result of hydrogen generation at low electrode potentials.
The electrochemical performance, meanwhile, benefits in that the electrochemical potential of the electrodes responds similarly to changes in temperature, humidity and poisoning, so keeping the background signal to a minimum. However, such a construction suffers from two disadvantages. Firstly, all three electrodes contain costly, highly active, noble metal catalyst, such as platinum. Secondly the potential of the reference electrode responds to exposure to an analytc gas, and must be isolated from incoming gas. This is currently achieved by a xe2x80x9creverse sealxe2x80x9d, a process which heat scals the area between the reference and working electrodes to the diffusion barrier below. This operation can be difficult to carry out reliably, and, more significantly, prevents the cell housings being built from only two pieces.
According to a further aspect of the present invention there is provided a gas sensor comprising: a sensory cell having at least one sensing electrode, a counter electrode, a sensory circuit; a test cell comprising at least a generating electrode, a counter electrode and a test circuit, the sensor being arranged so that in use an electrolyte is in contact with the electrodes, there being a membrane, through which gas may pass to the sensing electrode; the gas sensor being capable of operation in either a sensing mode, or in a test mode, characterised in that at least the generating electrode is coated with a catalyst for improving the efficiency of generation of a test gas.
The efficiency of the generation of hydrogen test gas is improved by making the gas generation electrode from a material which is a poor catalyst for the reduction of oxygen.
Preferably the catalyst comprises ruthenium dioxide and the test gas generated is hydrogen. The catalyst may be applied to the electrode(s) in the form of an ink. The invention provides a cheap and accurate means of enabling production of a high quality, efficient electrodes for use in a gas sensor.
Preferably electrodes are planar. One or more sensing electrodes and one or more generation electrodes may be enclosed in the same housing, in contact with common or separate electrolytes, with associated counter and reference electrodes as required by an embodiment, so that gas generation electrodes are close to sensing electrodes. This minimises the amount of gas that is need to effect the test. In one embodiment the test gas is delivered to the sensing electrode in the gas phase, by evolution into a communicating space above the electrodes.
In a preferred embodiment test gas to be generated is hydrogen and is generated by the reaction described in Eqn 1. It is a feature of this reaction that, if the generation electrode has a source of oxygen available, then oxygen will be reduced according to Eqn 2. It is apparent from Eqn 2 that electric current flows in parallel with that passed in the generation reaction described in Eqn 1, and so reduces the operating efficiency of the generator. The oxygen reduction reaction occurs if the generation electrode is permeable and in contact with the atmosphere. If the electrode is of an active catalyst such as those conventionally used in electrochemical sensors, for example platinum, the oxygen reduction current will be large compared with the hydrogen generation current at low electrode potentials. In the counter and reference electrodes alternative materials to platinum may be used.
Where the counter electrode is replaced, the alternative material requires a degree of catalytic activity such that it is able to perform the counter reaction without any loss of response of the overall cell. For example, in a cell which detects carbon monoxide by oxidation at its sensing electrode, oxygen is reduced at the counter electrode. Materials which are proposed for this purpose are gold, ruthenium oxide and carbon.
In the situation where the reference electrode is replaced with an alternative material, the requirement for the material is that its electrochemical potential is stable with time and that it varies in response to changes in temperature and humidity by a similar amount to the sensing electrode. Similar materials to the above are suggested as replacements for the noble metal catalyst.
The invention has significant worth if it is applied to cells which contain a self test function. In the manufacture of such cells, a second electrode print is required to produce the self test gas production electrodes. At least one of the self test electrodes must be a poor oxygen reduction electrode such that hydrogen is evolved in preference to other reactions. Suitable materials for these electrodes are again, ruthenium oxide, gold and carbon. Thus if a single material from this list can satisfy the requirements for the self test, counter and reference electrodes, significant reduction in catalyst expense can be achieved without adding a further screen printing stage.
Use of electrode materials for gas generation electrode(s) different to those used for sensing electrodes helps to improve the overall efficiency. The generation electrode is advantageously a poor catalyst for oxygen reduction. An example is (Ruthenium Dioxide), which is highly conductive, is easily dispersed in ink, and generates hydrogen in the presence of oxygen at a much lower electro-potentials (i.e. lower total current density) than may be platinum. Other materials which are also poor oxygen reduction catalysts may be used.
The electrode membrane is preferably a double print, with the sensing and reference areas of the membrane in platinum ink, and the generator electrodes and the sensor counter electrode(s) in ruthenium dioxide ink. Ruthenium dioxide ink may also be used as a conductive support layer to the platinum ink in order to increase conductivity and reduce cost.
Different aspects of the invention have been described and those are illustrated below by way of examples. However, it is understood that although some aspects are illustrated independently one from another, an embodiment incorporating two or more of the aspects described is envisaged within the scope of this description. That is to say for, example the aspect of the invention which includes baffles for reducing electrical cross-talk, may also have a capacitive energy storage circuit and electrodes with ruthenium dioxide catalyst. Similarly after combinations of features may be combined to provide a superior self test gas sensor.
In order to reduce oxygen access to the generator electrode if a membrane with a single print of platinum or other active oxygen reduction catalyst is preferred, the porosity of the membrane above the generation electrode may be reduced, but not to such a degree as to inhibit excessively the exit of hydrogen gas. This can be achieved, for example, by hot-pressing the membrane in the area of the generation electrode, by partially impregnating the area with PTFE or a similar impervious substance, or by sealing (either partially or completely) a low porosity material over the membrane. Alternatively, access of oxygen may he inhibited, while allowing hydrogen to exhaust from the sensor, by ensuring the exit through which hydrogen exhausts is narrow.